Irritable bowel syndrome is a clinically heterogeneous disorder with a worldwide prevalence of 10-20% (Longstreth et al., Gastroenterology, 130:1480-1491 (2006)). According to the Rome III criteria, patients with IBS can be categorized into four symptom subtypes based on the stool consistency: diarrhea predominant (IBS-D), constipation predominant (IBS-C), mixed subtype (IBS-M) with alternating episodes of both diarrhea and constipation, and unsubtyped IBS (IBS-U).
Current research has implicated the gastrointestinal microbiota and the brain-gut axis in the pathophysiology of IBS. Abdominal pain and discomfort associated with IBS is connected to the brain-gut axis and the response to stress hormones. Studies have shown that gastrointestinal microbiota of IBS patients is altered from that of healthy controls. There is also evidence that gastrointestinal microbiota causes post-infectious IBS (PI-IBS). IBS has been associated with aberrant gastrointestinal microbiota, and even bacterial overgrowth (Kim et al., Digestive Diseases and Sciences, May 2012).
Flagellin, the primary structural component of bacterial flagella, has been shown to activate both the innate and adaptive immune system in individuals. For example, antibodies against bacteria flagellin (A4-Fla2 and Fla-X) have been detected more frequently in patients with IBS than in healthy controls (p=0.004 and p=0.009, respectively; Schoepfer et al., Neurogastroenterol. Motil. 20:1110-1118 (2008)).
There is a growing body of evidence supporting the role of gut microbiome, stress hormones, inflammatory cytokines, and mast cell markers in various intestinal diseases or disorders. For instance, the antibodies to OmpC, Cbir1, FlaX and Fla2 have been proven to be valuable biomarkers of inflammatory bowel disease (IBD). Subsets of antibodies to Escherichia coli K12 proteins (e.g., Era, FocA, FrvX, GabT, YbaN, YcdG, YhgN, YedK, and YidX) can be used to distinguish between individuals with Crohn's Disease (CD) and healthy controls, and between individuals with CD and ulcerative colitis (Chen et al., Mol. Cell Proteomics, 8:1765-1776, (2009)). Individuals with post-infectious small intestine bacterial outgrowth (SIBO) associated with IBS which is often caused by infection from Campylobacter jejuni (C. jejuni, Cj), Escherichia coli (E. coli, Ec), Salmonella enteritidis (S. enteritidis, Se), Shigella flexneri (S. flexneri, Sf), may possess antibodies against flagellin proteins of the infecting bacteria (Spiller R and Garsed K., Gastroenterology, 136:1979-1988 (2009)).
Interestingly, treatments that target gastrointestinal microbiota such as antibiotics, probiotics and prebiotics appear to alleviate the symptoms of IBS. For instance, the antibiotic rifaximin appears to affect gut bacteria and reduce bacterial products that negatively affect the host individual.
In addition to the gut microbiome, mast cells also play an important role in the pathogenesis of IBS. Increased mast cell infiltration and activation in distal gut segments are associated with symptom onset and severity of IBS. These cells are also implicated in the elevated response of visceral afferent nerves to mucosal stimulus in IBS patients. Mast cell hyperplasia is commonly observed following infection by these bacteria in both post-infectious IBS and non-post-infectious IBS. Measurement of mast cell markers such as β-tryptase, histamine and prostaglandin E2 (PGE2) have important implications in the clinical diagnosis of IBS. Detailed methods of using mast cell markers to aid in the diagnosis of IBS are described in U.S. Pat. No. 8,114,616 and U.S. Patent Publication No. 2012/244558, the disclosures of which are hereby incorporated by reference in their entireties for all purposes.
IBS patients typically experience abnormal gut motility and visceral hypersensitivity mediated by the brain-gut axis or central stress response system. One arm of the brain-gut axis is the central efferent pathway, which is formed by the sympathetic nervous system and the hypothalamic-pituitary-adrenal axis (HPA). In stress-sensitive disorders including IBS, stress hormones of the HPA axis, such as adrenocorticotropin hormone (ACTH), cortisol, and catecholamine are released. Some studies have shown that the HPA axis response in IBS patients is caused by increased mucosal immune activation, which in turn increases plasma cytokine levels to stimulate the HPA axis. It has been theorized that a blunted stress response contributes to the symptoms of IBS. Moreover, symptoms of IBS due to increased expression of mast cell markers and altered microbiota antigen/antibody composition are thought to be exacerbated by an altered immune response caused by a dysregulated brain-gut axis.
In view of the foregoing, there is a need in the art for methods and kits that for differentially diagnose IBS from non-IBS samples. The present invention satisfies this and other needs.